With recent advances in nanoelectronics, products are being developed which apply physical phenomena unique to magnetic materials with minute sizes. Of these, there have been particularly rapid advances in the field of spin electronics, which utilize the spin of free electrons in magnetic materials.
In the field of spin electronics, spin valve elements utilizing the tunneling magneto-resistance (TMR) effect occurring in a layered structure of a ferromagnetic layer, an insulating layer, and a ferromagnetic layer in order, or utilizing the giant magneto-resistance (GMR) effect occurring in a layered structure of a ferromagnetic layer, nonmagnetic layer (conducting layer), and a ferromagnetic layer in order, are currently regarded as having the greatest possibility of application.
FIG. 7 and FIG. 8 are cross-sectional views showing the configuration of spin valve elements of the prior art. Of these, FIG. 7 shows the basic constituent portions of a spin valve element utilizing TMR. This spin valve element has a configuration in which a single insulating layer 24 and ferromagnetic layers 23 (fixed layer) and 25 (free layer) sandwiching the insulating layer are formed on a substrate 5; to this are further added, as necessary, electrode layers 21, 27, an antiferromagnetic layer (pinning layer) 22, a capping layer 26, and similar. The direction of the magnetization of the fixed layer 23 is fixed by magnetic coupling with the antiferromagnetic layer 22 and similar. When electrons are passed from the fixed layer 23 toward the free layer 25 in this element, a torque acts to cause the magnetization of the free layer 25 to be aligned parallel to the direction of the magnetization of the fixed layer 23. And conversely, when electrons are passed from the free layer 25 toward the fixed layer 23, a torque acts on the magnetization of the free layer 25 so as to be antiparallel to the direction of the magnetization of the fixed layer 23. Through these actions, the direction of magnetization of the free layer 25 can be controlled by the direction of current in the free layer 25. This phenomenon is called spin transfer magnetization reversal. For reasons explained below, in conventional structures the size in in-plane directions must be made very small (approximately 150 nm or less), so that electron beam exposure or other expensive equipment is used. In order to suppress the exchange coupling due to the leakage magnetic field from the film edge portions of the ferromagnetic layers 23 (fixed layer) and 25 (free layer) sandwiching the insulating layer 24, the portion on the upper side of the insulating layer 24 is generally formed to be sufficiently smaller than on the substrate side, and an insulating film 30 is generally formed on the periphery. A number of methods may be used to form these structures; for example, after forming the layered film from the substrate up to the electrode 27, a negative resist is applied and photolithography is used for exposure, after which ion milling is performed to expose the upper portion of the insulating layer 24, after which an insulating layer 30 is formed by covering with SiO2 or other means, followed by lift-off and a formation of the electrode 27 to be used for wiring.
FIG. 8 shows the basic constituent components of a spin valve element utilizing GMR. A difference with the element utilizing TMR in FIG. 7 is that the insulating layer 24 is replaced with a nonmagnetic layer 51; otherwise the functions are basically the same.
Among applications of these technologies, magnetic random access memory (MRAM) is attracting the greatest attention, and is anticipated as a replacement for conventional DRAM (dynamic random access memory) and SDRAM (synchronous DRAM).
Further, it is known that if a current and an external magnetic field are simultaneously applied to these spin valve elements, microwave oscillation occurs (see, for example, S. I. Kiselev, et al, “Microwave oscillations of a nanomagnet driven by a spin-polarized current”, Nature, Vol. 425, p. 380 (2003)). As an example, with respect to current direction, suppose that a current is passed such that the torque acts on the magnetization of the free layer 25 so as to become antiparallel to the direction of the magnetization of the fixed layer 23, and with respect to an external magnetic field, suppose that a torque acts on the magnetization of the free layer 25 so as to become parallel to the direction of the magnetization of the fixed layer 23. In this case, under conditions in which the two torques are counterbalanced, high-frequency oscillation in the microwave region can be induced.
In addition, it has been reported that when two elements are formed adjacently and when currents and external magnetic fields appropriate to these are applied, the oscillation frequencies and phases of the two become coincident, the frequency width is decreased, and microwave output at this time is also increased (see, for example, S. Kaka, et al, “Mutual phase-locking of microwave spin torque nano-oscillators”, Nature, Vol. 437, p. 389 (2005); F. B. Mancoff, et al, “Phase-locking in double-point-contact spin-transfer devices”, Nature, Vol. 437, p. 393 (2005); J. Grollier, et al, “Synchronization of spin-transfer oscillator driven by stimulated microwave currents”, Physical Review B73, p. 060409 (2006)). This phenomenon is called a phase locking phenomenon, and the mechanism, though not yet clarified, is inferred to arise from interaction between the high-frequency magnetic fields generated by each of the elements; this phenomenon is attracting attention as means of increased output.
The oscillation output of the above microwave oscillator elements is, in numerous reports, approximately 0.16 μW for TMR, and remains at approximately 10 pW for GMR, which are very low levels for practical application. Hence in order to obtain higher output, numerous minute elements must be integrated.                (2006)        
However, when using a spin valve element for microwave oscillation as described above, due to the Joule heat generated by the current, the element may be overheated. That is, because a single spin valve element is small, measuring approximately 100 nm on a side, the current density is high, and so Joule heat tends to cause local overheating. Because of this heat, material oxidation and other degradation tend to occur, and there has also been the problem that element failure readily occurs. Particularly in the case of elements using TMR, because current flows in an insulating layer of MgO or similar by the tunnel effect, the Joule heat at this time is extremely great compared with other layers. Moreover, when numerous elements are connected in series or in parallel and integrated as described above, the heat generation density increases, and these problems become still more serious.
Further, an actual example is here used to describe a problem. The impedance of an entire spin valve must be matched to a prescribed impedance in order to suppress high-frequency transmission losses. In the microwave region, input/output impedances are generally set at 50Ω. For example, by parallel-connecting 20 TMR spin valve elements each element of which is approximately 1 kΩ, an overall element impedance of 50Ω can be obtained. Such parallel connection is for example achieved by using an alumite minute hole structure and forming electrodes. Twenty elements are formed in mutual proximity, and so may be regarded as thermally coupled. Upon applying a voltage of for example 0.5 V to each element, the driving current per element is approximately 0.5 mA, and the input power is 0.25 mW per element. Because 20 elements are present, for the element as a whole the driving current is 10 mA, and as much as 5 mW of power is input. As one example, if 5 mW of power is input continuously for 10 nsec, and Cu measuring 100 nm diameter×100 nm thick (heat capacity 3.4×106 (J/m3K)) is heated in an adiabatic state by the resulting Joule heat, then the temperature increase reaches 1873 K. Such a large temperature increase causes failure of the spin valve element. Even when the temperature increase is not this great, oxidation and other degradation of the material may occur, and changes in magnetic characteristics dependent on temperature may give rise to instability in the spin valve element characteristics during operation. Hence there is a need to suppress increases in the element temperature insofar as possible. In particular, temperature increases are already a major problem in TMR elements with high electrical resistance, and as development advances and TMR elements with a high MR ratio (change in resistance upon magnetization reversal) are obtained, the average electrical resistance will also be higher, so that even more serious problems will result.
In this way, a method is sought in order that, during current driving of a spin valve element, by suppressing the temperature increase occurring due to the input power, degradation of spin valve element materials arising from the temperature increase and instability of the magnetic characteristics can be prevented, and the spin valve element can be driven with stability.